1. Field of the Invention
This invention relates to a composite filter advantageously employable in dark field observation systems and further relates to an illuminating device provided with the composite filter.
2. Description of Prior Art
A technology of detecting an image in dark field such as at dark night with no moonlight has been recently developed. This detecting technology utilizes starlight as light source and its detection system contains a multi-channeled plate and a photoelectric conversion element matching with the starlight.
It is known that the earth goes through a space in which approx. a hundred thousand stars are dispersed, for one day. Most of the stars emit light. Among them, ultraviolet (UV) rays emitted by new-born stars (age: 100,000-1,000,000 years old) are scattered or absorbed by an atmospheric gas, etc. so that most of the UV rays hardly reach the surface of the earth. On the contrary, the near infrared(IR) rays emitted by stars of approx. 5.times.10.sup.6 years old reach the surface of the earth. The spectrum of light emitted by the stars and received by the earth is shown in FIG. 1 as curve (i). Accordingly, photoelectric conversion elements employed in the detection system in dark field observation have photoelectric conversion characteristics as shown in FIG. 1 as curve (ii). This means that a light having a radiation spectrum in the region of 600 to 800 nm (or up to 900 nm) is multiplied by the photoelectric conversion element.
The observation system utilizing the starlight has been developed and now employed generally in the military and police works, because such observation system advantageously does not need employment or artificial light.
Since the observation system now utilizes a multi-channeled secondary electron multiplier, the gain is made at very high level. It is known that the detection device employed for the dark field observation system contains a number of artificial light sources such as a lighting device, various illuminating display devices, etc., most of which emit light having a radiation spectrum overlapping in part with the radiation spectrum of starlight. Particularly, the radiation spectrum of a red lamp (which is generally employed as a light source) well overlaps with the radiation spectrum of moonlight. Generally, the luminance of light emitted by these artificial light sources is several millions times to several ten millions times as high as that of the starlight. Therefore, if any light of the artificial light source is incidentally received in the visual field of the observer, not only such light causes noise but also the highly amplified artificial light causes burning and further destroys retinas of the observer's eyes.
For obviating such troubles, the present inventor invented a composite filter for intercepting light of a radiation spectrum in the region of 600-900 nm comprising at least one organic film filter and at least one glass filter, in which the organic film filter absorbs light of a radiation spectrum in the region of 600-700 nm and the glass filter absorbs light of a radiation spectrum in the region of 700-900 nm. This invention is described in U.S. Pat. No. 4,616,902.